Infrared imaging devices utilizing infrared radiation can perform capturing day and night, and infrared radiation can pass through smoke and mist at a higher rate than visible light so that temperature information of a subject can be obtained. As such, infrared imaging devices have a broad application range including the defense field, the security field and the like (for example, as surveillance cameras and fire detection cameras).
Quantum infrared solid-state imaging devices are mainly used, and conventionally, a cooling mechanism is required in such quantum infrared solid-state imaging devices for low-temperature operation. However, in recent years, non-cooling infrared imaging devices which do not require such a cooling mechanism have been developed.
Such a non-cooling infrared imaging device includes a semiconductor substrate, a read wiring portion formed on the semiconductor substrate, a support structure which is disposed over a recess formed in a surface portion of the semiconductor substrate and has connection lines which are electrically connected to the read wiring portion, and a cell portion which is disposed over the recess and supported by the support structure. The cell portion has an infrared absorption layer for absorbing incident infrared radiation, and plural thermoelectric conversion elements, which are electrically connected to the support structure, are electrically insulated from the infrared absorption layer, and generates an electrical signal by detecting a temperature variation in the cell portion.
In the above non-cooling infrared imaging device, since a signal is generated based on heat generated in the cell portion through infrared radiation absorption, it is important to hold such heat without leakage. To this end, as described above, the cell portion is spaced from the semiconductor substrate and supported only by the support structure, and the neighboring atmosphere is kept in a medium-level vacuum state of 1 Pa or lower to enhance the heat insulation performance of the cell portion. To maintain such an atmosphere, the device is sealed in a vacuum atmosphere after being mounted in a package. However, it is difficult to keep, for a long time, the internal pressure of the sealed package at a value immediately after the sealing. That is, the internal pressure of the package may increase with the passage of time because various gasses are generated from materials located inside the package including the materials of the infrared imaging device, and slight leakage occurs through the sealing portion due to the difference between the pressures inside and outside the package. Although it is possible to suppress increase in internal pressure by disposing a getter for absorbing generated gasses inside the package, it is not always possible to keep the internal pressure at a desired value merely by such getter. If the internal pressure of the package increases, the heat insulation performance of the cell portion is degraded. As a result, the responsivity of the infrared imaging device is lowered, and it comes not to perform a prescribed function.
In view of above, the internal pressure of the package may be monitored. One example method is to mount a micro-Pirani element in a juxtaposed manner and measure an internal pressure based on its output. A micro-Pirani element may be formed in the same chip as the infrared imaging device. However, in these methods, a processing circuit is required in addition to a processing circuit for processing an output signal of the infrared imaging device, and thus, for example, the circuit scale is increased, electrical interference may occur between the infrared imaging device and the micro-Pirani element when they operate simultaneously, and the number of pads of the package is increased to decrease package options.